Terahertz optoacoustic detection of aqueous salt solutions

Summary Terahertz radiation has been used to detect aqueous salt solutions; however, strong absorption of water in terahertz regime limits the application of traditional terahertz techniques. Here, we present a novel method in analyzing aqueous salt solutions via terahertz optoacoustics. Terahertz optoacoustic signals can be manipulated by temperature control, which allows the dampening of water background and providing more information of solute. We demonstrate that dynamic and continuous terahertz optoacoustic detections of salt solutions with different solutes, concentrations, temperatures, and terahertz spectral frequencies shows the significant potential of this method in distinguishing different salt solutions and quantitatively analyzing salt concentrations. Terahertz optoacoustics may be a powerful tool for quantitative and label-free detection of aqueous salt solutions to further study the complicated aqueous solutions in terahertz regime.


INTRODUCTION
Aqueous salt solution, in which ions dissolved in water produce considerable perturbation of the hydrogenbond structure of the liquid and bind to water molecules to form hydration shells (Drt, 1982). Salt solutions have specific physical and chemical characteristics, because ion hydration affects the structure and dynamics of water. Since the 19th century, the study on Hofmeister series describes the ability of ions to destabilize (salt out) or stabilize (salt in) egg white and serum proteins in aqueous solutions (Hofmeister, 1888;Zhang and Cremer, 2006). Many researchers believe that the Hofmeister series reflects the long-range structuring of water by specific ions kosmotropes (structure makers) versus chaotropes (structure breakers) (Marcus, 2009). However, there are some studies that suggest that the ions may be treated as simple defects in the water H-bond network, therefore cannot be characterized as either kosmotropes or chaotropes (Schmidt et al., 2009). These controversial opinions show that obtaining a complete understanding of salt solutions still remains challenging due to the physical and chemical complexities involved in it. Complementary techniques have been applied to study the salt solution, such as neutron and X-ray diffraction (Mancinelli et al., 2007;Gaspar et al., 2004), X-ray absorption spectroscopy (Cappa et al., 2006;Klewe and Pedersen, 1974;Reddy et al., 2016), Infrared and Raman spectroscopy (Smith et al., 2007;Bergstroem et al., 1991) and dielectric spectroscopy (Buchner and Hefter, 2009;Buchner et al., 1999;Chen et al., 2003;Lyashchenko and Lileev, 2010). In consideration of that the spectral response of hydrogen-bond network is in terahertz (THz) regime, THz radiation (wavelengths between 0.03 and 0.3 mm) has been used to analyze the characteristics of water and salt solutions (Tonouchi, 2007;Jin et al., 2020;Funkner et al., 2012;Schwaab et al., 2019).
THz spectroscopy is capable of probing collective motions that hydrogen bonds are formed and broken on picosecond time scales, offering a useful tool to study the features of hydration network in aqueous solutions (Schmidt et al., 2009;Mc Intosh et al., 2012;Ueno and Ajito, 2008;Takahashi, 2014). Several studies have utilized the time-domain THz spectroscopy to obtain THz absorption information of salt solutions in liquid state and frozen state, in order to explore the signatures of ion-water systems (Chen et al., 2020). However, the applications of THz technique on aqueous solutions face tough challenges caused by the strong absorption of water in the THz regime. Although THz reflectance spectra or transmissive THz spectroscopy with extremely thin samples have been employed to reduce background water (Brun et al., 2010), the complicated pretreatment of samples limits their dynamic and precise detection of aqueous solutions. In addition, strong absorption of water in aqueous solutions drowns out weaker signals In our recent work, we present a novel method in analyzing aqueous salt solutions via time-domain terahertz optoacoustics (THz-OA), breaking the limitation of complicated pretreatment of aqueous solution samples and dampening the strong optoacoustic signals of water background . Here, we further combine time-domain THz-OA and frequency-domain THz-OA to study the character of aqueous solutions of nine alkali halides and three alkaline earth metal halides, and figure out the relationship between THz-OA signal and the relative physical parameters, such as solute concentration, temperature and THz spectral frequency. Dynamic and continuous THz-OA detections of salt solutions show the significant potential of this method in distinguishing, quantitatively detecting different salt solutions with low concentrations by reflecting their THz absorption.

RESULTS
The time-domain THz-OA system, as shown in Figure 1, presented here incorporates terahertz radiation source, sample holder, temperature control module, piezoelectric ultrasonic transducer (system details see STAR Methods). The THz-OA signals are detected by ultrasonic transducers after the interaction between terahertz radiation and samples. The aqueous salt solutions to be detected are circulating through customized microfluidic chips in order to realize dynamic and continuous detection at the selected temperature.
First, we detected the time-domain THz-OA signals of pure water and salt solutions of nine alkaline halides and three alkaline earth metal halides, including monovalent and divalent chloride salt solutions, sodium and potassium salt solutions (as shown in Figures 2D-2G). The concentration of these salt solutions was prepared to be 2 mol/L consistently. The monovalent chloride salts included different cations of Li + , Na + , K + , Rb + , and Cs + . Divalent chloride salts included different cations of Mg 2+ , Ca 2+ , and Sr 2+ . Sodium and potassium salt solutions are with different anions of Cl À , Br À , I À . To quantitatively compare the signal intensity of different salt solutions with that of pure water, the peak-to-peak values of optoacoustic signals were extracted (inset in Figure 2A). Figure 2A shows that the THz-OA signal intensities of monovalent and divalent chloride salt solutions increase in the order of Li + < Na + < K + < Rb + < Cs + and Mg 2+ < Ca 2+ < Sr 2+ , respectively. For sodium and potassium salt solutions, the THz-OA signal intensities show a similar regularity of Cl À < Br À < I À (Figures 2B and 2C). The results demonstrate that the time-domain THz-OA signal intensities of different cations with the same anion or different anions with the same cation in salt solutions vary in consistency with the cation's or anion's position in elements groups of the periodic table.
To investigate the relationship between solute concentrations of salt solutions and time-domain THz-OA signals, we presented the detection of monovalent and divalent chloride salt solutions, sodium and

OPEN ACCESS
potassium salt solutions with different solute concentrations ranging from 1-5 mol/L at an interval of 1 mol/L. The peak-to-peak values were extracted from THz-OA signals ( Figure S1) of different salt solutions and compared versus solute concentrations. It is obvious that the intensities of THz-OA signals increase at a near-linear trend as the increasing of solute concentrations for all kinds of salt solutions ( Figure 3). In addition, the slopes of linear fittings are following the order of LiCl < NaCl < KCl < RbCl < CsCl, MgCl 2 < CaCl 2 < SrCl 2 , NaCl < NaBr < NaI, and KCl < KBr < KI, respectively.
The previous study has shown that the time-domain THz-OA signals of water could be manipulated through temperature regulation, which can be dampened at low temperature in order to allow sensitive detection of low concentration aqueous salt solutions . To further explore new features of salt solutions with dampened water background, we measured monovalent and divalent chloride salt solutions and sodium salt solutions with the solute concentration of 2 mol/L by THz-OA method at low temperatures. The intensities of THz-OA signals at 6 C are displayed in Figures 4A-4C. Different from the result at room temperature, the THz-OA signals with maximum intensities are from NaCl, CaCl 2 and NaI solutions, respectively. In addition, LiCl, CaCl 2 , and NaBr solutions at concentrations of 1-5 mol/L were detected to obtain the intensity differences of their THz-OA signals at temperatures between 20 C and iScience Article 6 C. Figures 4D-4F show that those differences decrease with increasing solute concentration, which may be caused by the different proportions of water in the solutions with different solute concentrations. When the concentration rises, less proportion of water influenced by lowering temperature leads to the smaller intensity differences of THz-OA signals between high and low temperatures.
To further explore the capability of temperature-controlled time-domain THz-OA detection of salt solutions, we conducted measurements at low temperatures (4-10 C) on NaCl solutions with low concentrations of 0.01, 0.02, 0.03, and 0.04 mol/L, similar to the NaCl concentration in human body (Wishart et al., 2018). Figure 5 shows the extracted peak-to-peak values of THz-OA signals of NaCl solutions and pure water. Linear fitting was used to figure out the muting temperature of each solution (Prakash et al., 2020;Xu et al., 2021). The muting temperatures of aqueous solutions with NaCl concentrations of 0.01, 0.02, 0.03, and 0.04 mol/L were calculated to be 3.73, 3.40, 3.23, and 3.01 C, respectively. A linear fitting with a slope of À23.47 C/(mol/L) and a linear fitting R-square of 0.9807 is acquired (inset in Figure 5).
Finally, to study the THz absorption of aqueous salt solutions at another THz frequency, we apply frequency-domain THz-OA method to detect pure water and NaCl solutions with relatively low concentration (0.2, 0.4, 0.6, 0.8, mol/L) at 4 C by noncontact measurement. The frequency-domain THz-OA system, as shown in Figure 6A, presented here includes terahertz radiation source, sample cell, temperature control module, and microphone (system details see STAR Methods). The NaCl solution in the cell is illuminated by sinusoidal modulated continuous-wave THz radiation and generates THz-OA signal with the same modulation frequency. Figure 6B shows that the THz-OA signal intensities at 4 C decrease with the increase of NaCl concentration, which corresponds to the trend of absorption coefficient in previous studies (Vinh et al., 2015;Jepsen and Merbold, 2010).

DISCUSSION AND CONCLUSION
We present the THz-OA detection of aqueous solutions of nine alkali halides and three alkaline earth metal halides. Microfluidic chips applied in time-domain THz-OA systems enable dynamic and continuous   THz-OA signals of aqueous solutions can be manipulated by altering temperature . By lowering the temperature of salt solutions, it is able to mute the THz-OA contribution from water background ( Figure 4) and enrich for the contribution from solutes based on Equation (4). Different from traditional THz spectroscopy whose signals of aqueous solutions is mainly contributed by water, the proposed method can extract information of interested solutes through dampening the THz-OA signals of water. This water-manipulated THz-OA method can uniquely achieve sensitivity-enhanced detection of salt solutions with ions' concentration reaching the concentration level in the human body. The THz-OA signals with maximum intensities at low temperatures are from salt solutions containing important ions in life activities, demonstrating the THz-OA method has the great potential to be a powerful tool in biological applications. In addition, the relationship between muting temperatures and salt solutions with low concentrations could be adopted to implement label-free quantitative detection.
Previous studies have shown that the THz absorption of salt solution is concentration and frequency dependent (Jepsen and Merbold, 2010). The THz absorption coefficient of salt solution can be converted by the complex dielectric function through Equation (6), and the complex dielectric function can be described by the Debye model as Equation (5). In Figure 6B, the variation trend of frequency-domain THz-OA signal intensities versus NaCl concentration is consistent with that of absorption coefficient at 0.1 THz in Ref (Vinh et al., 2015) and (Jepsen and Merbold, 2010), which demonstrates that THz-OA method can reflect the THz absorption and can be further used for quantitative spectral analysis in low concentration aqueous solution.
In conclusion, we demonstrate a novel THz-OA method for the dynamic and continuous detection of salt solutions, and investigate the influence of solute concentration, temperature and spectral frequency on THz-OA signal. The proposed method is capable to distinguish salt solutions with different solutes, quantitatively detect and reflect THz absorption of low concentration aqueous solutions. For further study, THz-OA spectroscopy will be developed for deeper understanding of THz absorption of aqueous iScience Article solution, and THz-OA microscopy will be studied for label-free and real-time imaging of solutes in aqueous solution.
Limitations of the study 1. In Figure 4, we choose THz-OA signals of aqueous salt solutions at 6 C instead of that at the watermuting temperature which is reported to be 4 C. Although the background signal of water at 6 C is more stable than that around the water-muting temperature, the influence of water absorption still remains for solutions detection.
2. Terahertz radiation source used in the present time-domain THz-OA system has a wide spectrum of 0.2-1.5 THz; therefore, the time-domain THz-OA signals cannot provide the characteristic spectrum of specific ions. By combining the tunable, narrow-spectrum terahertz radiation sources in the future, our time-domain THz-OA method will be able to identify different target molecules based on their THz-OA fingerprints.

STAR+METHODS
Detailed methods are provided in the online version of this paper and include the following:

ACKNOWLEDGMENTS
This work was supported by the National Key Research and Development Program of China (with grant NO. 2017YFA0701004), the National Natural Science Foundation of China (Grant No. 82171989, 61675145, 61722509, 61735012, 61420106006), and Tianjin Municipal Government of China grant 19JCQNJC12800.

AUTHOR CONTRIBUTIONS
J.L., Z.T., and W.Z. proposed the terahertz optoacoustics method. L.J., K.Z., and Y.Y. prepared the aqueous solution samples. L.J., Y.Y., and S.L. performed all the measurements. L.J., K.Z., Y.Y., and J.L. analyzed the measured data. J.L., Z.T., and W.Z. supervised the theory and the measurements. All the authors discussed the results and contributed to the writing of the manuscript.

DECLARATION OF INTERESTS
The authors declare no competing interests.

Sample preparation
All salts we used have the highest available purity and are listed in the key resources table. In order to prepare salt solutions with specific concentrations, the mass of solute is calculated according to the relative molecular mass. Then, the solute is weighed with a high-precision balance and completely dissolved with deionized water in a volumetric flask.

Principle of THz-OA
The generation and propagation of optoacoustic signal induced by a short electromagnetic pulse can be described by optoacoustic equation as (Wang and Wu, 2008) where V S is the speed of sound, C P is the specific heat capacity, b is the thermal coefficient of volume expansion, and H is the heating source. When the pulse width is much less than the thermal relaxation time and pressure relaxation time, the local pressure rise after the laser excitation pulse can be written as where G is the dimension-less Grü neisen parameter. The factor m a represents the optical absorption coefficient, which is determined by the absorption characteristics of the material at the given frequency of electromagnetic wave, F shows the optical fluence (the optical energy per unit area) and h th defines the percentage of absorbed energy converted into heat. Since the terahertz pulse we use in time-domain THz-OA has a spectral range, the local pressure should be exactly written as where Fv(v), m a (v) is the distribution of optical fluence and absorption coefficient at a given spectral frequency v, respectively. At 4 o C, due to b water = 0 for water, the local pressure of aqueous solution based on water muting method and Despretz law can be further written as (Prakash et al., 2020) where c is the solute concentration, and b 2 , K is a constant in thermal coefficient of volume expansion and Despretz law respectively, which are determined by aqueous solution. In different low concentration aqueous solution, the change of b 2 , K, V S , C P , h th and m a can be ignored (Wang and Wu, 2008). The local pressure at water muting temperature is proportional to the solute concentration and can be used for quantitative detection analysis.
The optoacoustic pressure of samples illuminated by terahertz pulses leads to the generation of optoacoustic signals. The THz-OA signal relates to the Grü neisen parameter, absorbing materials and characteristics of terahertz radiation (Ntziachristos, 2010). In aqueous solutions, these parameters are functions of both temperature and solute concentration (Darros-Barbosa et al., 2003). In this article, we use THz-OA method to measure salt solutions with different cation-anion pairs, different ion concentrations and temperatures.

Debye model
The complex dielectric function, εðvÞ = ε 0 ðvÞ + iε 00 ðvÞ, of polar liquids can be described by Debye model as (Vinh et al., 2015) εðvÞ where ε N is the high-frequency contribution to the complex dielectric function, Dε j , t j is the dielectric strength and relaxation time of the jth relaxation process respectively, s s is the static conductivity of the electrolyte solution. The relation between complex dielectric function and absorption coefficient is given as (Vij et al., 2004) m a ðvÞ = 4pv C 0 0 @ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi ε 0 ðvÞ 2 + ε 00 ðvÞ 2 p 2 À ε 0 ðvÞ 2 1 A 1=2

(Equation 6)
where C 0 is the speed of light in vacuum. The solute concentration has influence on dielectric properties and THz absorption of aqueous solution (Jepsen and Merbold, 2010). In order to figure out the effect of solute concentration on THz absorption, we measure NaCl solutions with different concentrations at water muting temperature via frequency-domain THz-OA method.

QUANTIFICATION AND STATISTICAL ANALYSIS
There is no statistical analysis or quantification in this paper.

ADDITIONAL RESOURCES
We have no relevant resources.